1. Field of the Invention
This invention relates to the protection of metallic equipment against corrosion, and more particularly to methods for protection using sacrificial anodes.
2. Description of the Art
Cathodic protection systems are employed to prevent corrosion of metal structures exposed to an electrolytic environment. Cathodic protection can be effected for marine or subterranean corrodible structures by electrically connecting the corrodible structure to sacrificial anodes constructed of a metal that is higher in the electromotive series than the protected structure, i.e., a metal that is anodic to the material of the protected structure. When the protected structure and the electrically connected sacrificial anode are both disposed within the same electrolytic environment (e.g., soil or water containing ions), a galvanic cell is formed in which the protected structure is the cathode.
Metal atoms on the exposed surface of the sacrificial anode are ionized by the surrounding electrolyte and go into solution with the electrolyte, thereby corroding the sacrificial anode. Due to the difference in electrical potential between the cathodically protected metal and the sacrificial anode, electrons produced by the electrochemical corrosion reaction of the anode flow as an electrical current through the electrical connection between the sacrificial anode and the protected structure. When electrons reach the protected structure, they combine with positive ions (such as hydrogen ions) or dissolved oxygen in the electrolyte at the surface of the protected structure. The protected structure does not corrode since the positive ions or oxygen would otherwise initiate a corrosion reaction at the surface of the protected structure.
For protecting structures made from ferrous metals such as iron and steels, the sacrificial anodes are generally magnesium, aluminum, or zinc. Aluminum is a preferred material for this service, due to its relatively low price, low density, and high theoretical electrical capacity (due to formation of a trivalent cation). However, since pure aluminum quickly passivates during galvanic operation, because a layer of oxide material forms on the anode surface, it has been found necessary to utilize various alloys of aluminum.
A study of electrochemical efficiency, as a function of composition, for various aluminum alloy galvanic anodes in sea water was given by T. J. Lennox, Jr., M. H. Peterson, and R. E. Groover, Materials Protection, Vol. 33, February 1968, pages 33 through 37. This paper relates the former popularity of 5 percent zinc-aluminum alloys, and the test results of various zinc-aluminum alloys which contain other metals, selected from tin, mercury, boron, and iron.
U.S. Pat. No. 4,141,725 to Murai et al. describes galvanic anodes, which are aluminum alloys also containing zinc, indium, calcium, magnesium, and at least one rare earth element. Ranges of metal concentrations said in the patent to be useful can be described as: EQU 10%&gt;Zn&gt;0.5% EQU 0.05%&gt;In&gt;0.005% EQU 0.5%&gt;Ca&gt;0.005% EQU 4%&gt;Mg&gt;0.1% EQU 0.05%&gt;RE&gt;0.001%
wherein "RE" represents one or more rare earth metals and all percentages are expressed on a weight basis. The anodes are said to be useful for protecting steel structures in sea water.
Eberius, in U.S. Pat. No. 3,383,297, describes an anode for cathodic protection, which is an alloy of zinc with at least 0.02 weight percent rare earth metal. The rare earth can be lanthanum, lanthanum with up to 50 weight percent cerium, or misch metal containing at least 35 weight percent lanthanum. It is reported in the patent that adding the rare earth reduces polarization inactivation, corrosion, and coating of the zinc anodes.
Aluminum was alloyed with cerium or misch metal by Sarbey in U.S. Pat. No 3,373,779 and used for wire in flash lamps; the alloy was more easily ignited than was pure aluminum. Knapp et al., in U.S. Pat. No. 2,980,529, alloyed rare earth metals with aluminum, for avoidance of alumina formation when the aluminum is added to molten steel. The electrical conductivity of relatively impure aluminum for transmission line wire was improved by the addition of misch metal in U.S. Pat. No. 4,213,799 to Raghavan et al., and by the addition of yttrium in U.S. Pat. No. 4,213,800 to Mayo et al.